1. Field of the Invention
The present invention relates to a motion detection solid-state imaging device for detecting motion of an image.
2. Description of the Related Art
Among conventional solid-state imaging devices, those of the charge coupled device (CCD) type are dominant and are widely used in various fields. In a CCD type imaging device, light is photoelectrically converted by a pixel such as a photodiode or a MOS diode. The stored signal charge is guided to a highly sensitive charge detection section via a CCD transmission channel, where it is converted into a voltage signal. The advantages thereof are, for example, a high S/N ratio and a high output voltage.
Moreover, a MOS type imaging device reads out signals from pixels by an X-Y address type scanning circuit. Such devices include those of a non-amplifying type which directly read out a signal charge of a pixel and those of an amplifying type which do not read out the signal charge itself but read out the signal charge after being amplified in the pixel.
FIG. 10 shows a pixel of the non-amplifying type solid-state imaging device. In the figure, a photodiode 91 photoelectrically converts incident light and stores the signal charge. A transistor 92 is turned on in response to a pulse signal .PHI..sub.X from a scanning circuit and transmits the signal charge of the photodiode 91 and outputs a signal voltage V.sub.sig. Since the signal charge of the photodiode 91 is drained via the transmission and readout operations, the pulse signal .PHI..sub.X effects both readout and reset operations of the signal charge.
In such a non-amplifying type solid-state imaging device, since the amount of signal charge is small, it is easily influenced by parasitic capacitance of a readout line, whereby the influence of a noise during a readout operation is significant.
On the other hand, in the amplifying type solid-state imaging device, since the signal charge is read out after being amplified in a pixel, the influence of noise during a readout operation is substantially negligible, thereby being more advantageous than those of the non-amplifying type in view of the S/N ratio. Moreover, there is no limit on the amount of signal charge to be read out from a pixel, whereby the device of this type is more advantageous than a non-amplifying type solid-state imaging device in terms of dynamic range. Furthermore, as will be described later, it is required only to drive horizontal and vertical signal lines, with the driving voltage being low, whereby the power consumption is less than that of a non-amplifying type solid-state imaging device.
Each pixel in such an amplifying type solid-state imaging device includes a photoelectric conversion section for generating a signal charge according to incident light and an amplification section for converting the signal charge from the photoelectric conversion section to a signal voltage and amplifying it. The pixels can be classified into those of a lateral type where the photoelectric conversion section and the amplification section are provided in a planar arrangement and those of a vertical type where they are provided in a three-dimensional arrangement.
As an example of the lateral type pixel, those of an APS type, as shown in FIG. 11, are known (see B. Ackland, et al., "Camera on Chips", ISSCC'96, pp. 22 to 25, February 1996).
In the figure, a signal charge generated in a photoelectric conversion section 101 is transferred to a gate of a transistor 102 so as to be an input voltage of the transistor 102. The transistor 102 is provided for impedance conversion and performs current amplification of a signal. The output of the transistor 102 is read out as a signal voltage V.sub.sig via a pixel selection transistor 103. During a drainage period after a readout operation, a reset transistor 104 is turned on so as to drain a signal charge stored in the gate of the transistor 102 to a drain V.sub.D.
As an example of a vertical type pixel, those of a CMD type, as shown in FIG. 12, are known (see Nakamura, et al., "Gate-stored MOS Phototransistor Image Sensor", Transaction of the Institute of Television Engineers of Japan, vol. 41, No. 11, pp. 1047 to 1053, 1987).
In such a pixel, an initial voltage is applied to the gate of a transistor 111, and a signal charge generated through photoelectric conversion is stored in the gate. During a readout period, a pulse signal .PHI..sub.X is applied to the gate of the transistor 111 so as to read out a signal voltage V.sub.sig of the transistor 111. During a drainage period, a pulse signal .PHI..sub.R greater than the voltage .PHI..sub.X is applied to the gate of the transistor 111 so as to drain the signal charge of the gate to a substrate (not shown). Therefore, all of these operations: photoelectric conversion, signal charge amplification and pixel selection are performed by the transistor 111.
However, the use of this pixel requires three different voltages to be selectively applied to the gate of the transistor 111, where at least one of the three different voltages is a high voltage.
In view of this, as an invention separate from the present invention, the applicant of the present invention has applied for a patent application on a pixel (Japanese Laid-Open Publication No. 8-78653), which can be driven by a low voltage. In this case, as shown in FIG. 13, a signal charge is stored in the gate of a transistor 121, to which gate a pulse signal .PHI..sub.X is applied so as to read out a signal voltage V.sub.sig. During a drainage period, a pulse signal .PHI..sub.R is applied to a transistor 122 so as to drain the signal charge to a substrate (represented by a ground symbol in the figure). Voltages .PHI..sub.X and .PHI..sub.R can both be made low. Thus, the pixel can be driven by two low voltages.
The respective configurations of the pixels illustrated in FIGS. 11 to 13 may be represented commonly by a schematic diagram such as one shown in FIG. 14. A photoelectric conversion section 131 not only performs photoelectric conversion but also outputs a signal voltage in response to a pulse signal .PHI..sub.X and drains a signal charge in response to a pulse signal .PHI..sub.R. An amplification section 132, when receiving a signal charge, amplifies the signal charge and outputs a signal voltage V.sub.sig.
A plurality of pixels having such a configuration are arranged in a matrix so as to form an imaging screen of a solid-state imaging device. A signal voltage of each pixel is obtained according to an image on the receiving field, and signal voltages obtained for the pixels are used as image signals. Moreover, a light integration period of each pixel is defined from a reset operation for emptying the signal charge of the pixel to a readout operation for reading out a signal charge stored in the pixel through the photoelectric conversion. The reset operation and the readout operation may coincide with each other in some cases.
A solid-state imaging device is not only used for imaging an image, but also used for detecting motion of an image on a receiving field of the solid-state imaging device. For example, a fixed scene can be continuously imaged so as to detect a human, or the like, entering the scene. In response to the entry of the human, various equipments can be controlled, or the entry itself can be notified or recorded.
In order to detect such motion of the image on the receiving field, a device such as that shown in FIG. 15 has been proposed.
In this device, in each of frame periods (F-1), F, (F+1), . . . , shown in FIG. 16, an image signal 144 for the frame period is output from a solid-state imaging device 141. The image signal 144 for the frame period is stored in a frame memory 142 and output to a differential amplifier 143. Moreover, the frame memory 142 stores the image signal 144 for the frame period and outputs an image signal 145 of the previous frame period to the differential amplifier 143. The differential amplifier 143 obtains and outputs the difference between the image signals 144 and 145 of the consecutive frame periods.
The calculation by the differential amplifier 143 is performed by each pixel. That is, a signal voltage of a pixel at one pixel address, e.g., (i.sub.th row, j.sup.th column), is obtained for each of the two consecutive frame periods so as to obtain the difference in signal voltages between these frame periods.
For example, if there is no change in the image on the receiving field over the consecutive frame periods (F-1) and (F), a signal voltage of a pixel does not change over the frame periods (F-1) and (F), whereby the difference between the signal voltages of the respective frame periods is zero. On the other hand, if there is some change in the image on the receiving field over the consecutive frame periods (F-1) and (F), as shown in FIG. 17, whereby the signal voltage of a pixel 146 at the pixel address (i, j) changes over the frame periods, then, the difference between the signal voltages of the respective frame periods is not zero.
Therefore, only when the image on the receiving field changes, (i.e., the difference between the signal voltages of the respective frame periods is not zero) does the output of the differential amplifier 143 vary. Thus, motion of the image on the receiving field can be detected.
However, in such a device, if a frame memory 142 stores analog signals, it is necessary to match the gain and linearity of the signal voltage output from the frame memory 142 to those of the signal voltage output from the solid-state imaging device 141. It is also necessary to sufficiently suppress the noise level. Consequently, it is extremely difficult to realize such a device. On the other hand, if a frame memory 142 stores digital signals, it is necessary to effectuate A/D conversion upon the signal voltage output from the solid-state imaging device 141 to obtain a digital signal, store the digital signal in the frame memory 142, and effectuate D/A conversion upon the digital signal output from the frame memory 142 to obtain an analog signal voltage. Thus, since it is necessary to provide an A/D convertor and a D/A convertor, the circuit scale increases, whereby some cost increase is unavoidable.
FIG. 18 illustrates another device for detecting motion of an image on a receiving field (see A. Dickinson, et al., "A 256.times.256 CMOS Active Pixel Image Sensor with Motion Detection", ISSCC95, p.226-227, February 1995).
In this case, a plurality of pixels 151 are arranged in a matrix, and a column amplifier 152 is provided for each of the vertical columns of the arrangement of the pixels 151. For each horizontal row, signal voltages of the pixels 151 in a row are transmitted to the respective column amplifiers 152. The outputs of these column amplifiers 152 are sequentially transmitted to a differential amplifier 153.
Each pixel 151 is configurated as illustrated in FIG. 19, where a signal charge generated by a photoelectric conversion element 154 is stored in a capacitor C.sub.P. A charge transmission transistor 155 is applied with a pulse signal .PHI..sub.T in the gate thereof and is thus turned on so as to transmit the signal charge to a capacitor C.sub.S. The gate of an amplification transistor 156 is applied with a voltage across the capacitor C.sub.S so as to output a signal voltage V.sub.sig according to the signal charge of the capacitor C.sub.S, from the amplification transistor 156. A selection transistor 157 is applied with a pulse signal .PHI..sub.V in the gate thereof and is thus turned on so as to output the signal voltage V.sub.sig of the amplification transistor 156 to the column amplifier 152. A reset transistor 158 is applied with a pulse signal .PHI..sub.R in the gate thereof and is thus turned on so as to drain the signal charge of the capacitor C.sub.S to a drain V.sub.D.
In the column amplifier 152, on the other hand, pulse signals .PHI..sub.sA and .PHI..sub.sB are applied at respective timings to the gates of the respective selection transistors 161 and 162 so as to turn on the selection transistors 161 and 162, thereby transmitting the signal voltage V.sub.sig from the pixel 151, via the respective selection transistors 161 and 162, to the respective capacitors C.sub.A and C.sub.B, where the signal voltages V.sub.sig are stored. At the same time, a pulse signal .PHI..sub.H is applied to each of the gates of transistors 163 and 164 so as to turn on these transistors 163 and 164, thereby sending the signal voltages V.sub.sig respectively stored in the capacitors C.sub.A and C.sub.B to a differential amplifier 167 via amplifiers 165 and 166, respectively.
The differential amplifier 167 obtains and outputs the difference between the signal voltages V.sub.sig respectively stored in the capacitors C.sub.A and C.sub.B.
FIG. 20 shows the respective timings of signals used in such a device. In this case, the duration of a frame period varies for different horizontal rows. The i.sup.th row will be discussed below.
As is apparent from the timing diagram, in the pixel 151, immediately before the end of the frame period (F-1) (when the signal charge for the frame period (F-1) is still being stored in the capacitor C.sub.S), a pulse signal .PHI..sub.V is activated so as to turn on the selection transistor 157, thereby outputting the signal voltage V.sub.sig (signal voltage 171 in FIGS. 19 and 20) for the frame period (F-1) to the column amplifier 152 via the selection transistor 157.
In the column amplifier 152, immediately before the end of the frame period (F-1), a pulse signal .PHI..sub.sB is activated so as to turn on the selection transistor 162, thereby transmitting the signal voltage V.sub.sig (signal voltage 173 in FIGS. 19 and 20) from the pixel 151 for the frame period (F-1), via the selection transistor 162, to the capacitor C.sub.B, where the signal voltage V.sub.sig is stored.
Then, in the pixel 151, a pulse signal .PHI..sub.R is activated so as to turn on a reset transistor 158, thereby draining the signal charge of the capacitor C.sub.S to the drain V.sub.D via the reset transistor 158, thus eliminating the signal voltage V.sub.sig for the frame period (F-1). Moreover, a pulse signal .PHI..sub.T is activated so as to turn on the charge transmission transistor 155, thereby transmitting the signal charge which has been stored in the capacitor C.sub.P since the beginning of the frame period F to the capacitor C.sub.S via the charge transmission transistor 155. Thus, the signal voltage V.sub.sig (signal voltage 171 in FIGS. 19 and 20) for the frame period F is output from the amplification transistor 156.
Next, in the column amplifier 152, a pulse signal .PHI..sub.sA activated so as to turn on the selection transistor 161, thereby transmitting the signal voltage V.sub.sig (signal voltage 172 in FIGS. 19 and 20) from the pixel 151 for the frame period F, via the selection transistor 161, to the capacitor C.sub.A, where the signal voltage V.sub.sig is stored.
Immediately after this, a pulse signal .PHI..sub.H is activated so as to turn on the transistors 163 and 164, thereby sending the signal voltages V.sub.sig (signal voltages 172 and 173 in FIGS. 19 and 20) stored in the respective capacitors C.sub.A and C.sub.B to the differential amplifier 167.
The differential amplifier 167 obtains and outputs the difference between the signal voltage V.sub.sig stored in the capacitor C.sub.B for the frame period (F-1) and the signal voltage V.sub.sig stored in the capacitor C.sub.A for the frame period F.
That is, immediately before the end of the frame period (F-1), the signal voltage V.sub.sig for the frame period (F-1) is transmitted from the pixel 151 to the column amplifier 152 so as to store the signal voltage V.sub.sig for the frame period (F-1) in the column amplifier 152. Subsequently, after the beginning of the frame period F, the signal voltage V.sub.sig for the frame period F is transmitted from the pixel 151 to the column amplifier 152 so as to store the signal voltage V.sub.sig for the frame period F in the column amplifier 152, and the difference between the signal voltages V.sub.sig for the respective frame periods (F-1) and F is obtained by the differential amplifier 167.
At this point, if the image on the receiving field is stationary, there is no change in the level of light incident upon the pixel 151 over the frame periods (F-1) and F, whereby the difference between the signal voltages V.sub.sig of the respective frame periods (F-1) and F is zero. On the other hand, if the image moves, the level of light incident upon the pixel 151 changes over the frame periods (F-1) and F, whereby there is some difference between the signal voltages V.sub.sig of the respective frame periods (F-1) and F. Therefore, it is possible to detect motion of the image on the receiving field based on the difference.
Such a transmission operation of the signal voltages V.sub.sig of the respective frame periods (F-1) and F from the pixel 151 to the column amplifier 152 and an output operation of the difference between the signal voltages V.sub.sig are performed for each of the pixels 151 in one horizontal row, and are repeated for each of the rows (i-1), i and (i+1).
However, the solid-state imaging device requires two capacitors C.sub.P and C.sub.S for storing two signal charges and a charge transmission transistor 155 between the capacitors C.sub.P and C.sub.S to be provided therein. Thus, the size of each pixel 151 increases.
As described above, in the conventional device shown in FIG. 15, if a frame memory 142 stores analog signals, it is necessary to match the gain and linearity of the signal voltage output from the frame memory 142 to those of the signal voltage output from the solid-state imaging device 141. It is also necessary to sufficiently suppress the noise level. Therefore, it is extremely difficult to realize such a device. On the other hand, if a frame memory 142 stores digital signals, since it is necessary to provide an A/D convertor and a D/A convertor, the circuit scale increases, whereby some cost increase is unavoidable.
Moreover, the other conventional devices shown in FIG. 18 require two capacitors C.sub.P and C.sub.S and a charge transmission transistor 155 to be provided therein. As a result, the size of each pixel 151 increases.